Telephone equalizer circuit



July 14, 1953 E 1. GREEN TELEPHONE EQUALIZER CIRCUIT Filed April 11,1950 II u m/vewron E l GREEN Br j ATTORNEY Patented July 14, 1953TELEPHONE EQUALIZER CIRCUIT Estill 1. Green, Short Hills, N. J assignorto Bell Telephone Laboratories,

Incorporated, New

York, N..Y., a. corporation of New York Application April 11, 1950,Serial No. 155,329

This invention relates to improvements in volume equalizer circuitswhich may be embodied in telephone station sets. Such sets areordinarily located at the telephone subscribers premises and are used bytelephone subscribers to communicate with distant parties.

An object of this invention is the improved equalization of a telephonestation set so that the reception and transmission amplitude levels oftelephone voice signals for said station set will besubstantiallyindependent of the length of transmission line connecting said stationset to the switching center.

. Another object of :this invention is the elimination of circuitcomponents in volume equalizer circuits which are susceptible tofrequent failure from ordinary subscriber station set use.

Another object of this invention is an equalizer circuit which employscircuit components whose design combines the features of long life,

mechanical ruggedness, and economy in cost.

A further object of this invention is the design of a volume equalizercircuit wherein the same circuit components may be used regardless ofthe distance between the subscribers substation Whose station set isto'be equalized and the telephone ofiice supplyingthe telephone talkingcurrent thereby simplifying the installation of equalizer circuitcomponents in the different subscriber substations served by aparticular telephone switching office.

As is well known, subscribers substationsets are located at variousdistances, within a permissible range, from a telephone switching officeand also in many cases conductors of difierent gauges are employed inthe interconnections. As

a result of these factors, as Well as others, there is a considerablevariation in the amplitude of signals received and transmitted by asubscriber unless some means are employed for compensating for thedifferences.

In a volume equalizer circuit disclosed in Patent 2,620,402, grantedDecember 2, 1952, to Botsford, Boysen, Aikens, Dietze, Goodale andInglis,

a tungsten filament having a positive temperature'coefiicient ofresistance and enclosed within a: gas-filled vessel is connected inseries with a subscribers transmitter. Since the resistance of thetungsten filament varies directly with the direct current value passingtherethrough and since the value of the direct current varies inverselywith the length of the loop, this arrangement equalizes the voicecurrents generated by the subscribers transmitter. In order to limitthelvoice current amplitudes in the receiver on 6 Claims. (Cl. 17981)short loops, a resistance element having a negative temperaturecoefficient of resistance is con nected in shunt with the receiver. Thisshunt resistance element is thermally coupled to the tungsten filamentby enclosing it within the vessel containing the tungsten filament. Withthis arrangement a short loop as compared to a long loop increases thecurrent passing through the tungsten filament thereby increasing theheat energy transfer to the shuntresistance element so as to equalizethe reception signal level.

In the circuit of the present invention, two negative temperaturecoefficient resistance elements are employed, one in shunt connectionwith the receiver and one in shunt connection with the between thetransmitter and the receiver in that the transmitter equalizin elementprovidesthe heat energy variations controlling the equalization of thereceiver.

A feature of l the circuit of this invention is that station setmaintenance problems are minimized because rugged elements capable ofwith standing severe mechanical shock and vibration are employed, andbecause failure of the shunt resistance elements does not preventoperation of the station set. In equalizer circuits employmg seriesresistance elements, a failure of an equalizer element will usually openthe transmitter or receiver circuit thereby preventing operation of thestation set.

Another feature of this inventionis the independent volume equalizationof a station set transmitter and receiver by the use of resistancecomponents which shunt said transmitter and receiver.

Another feature of this invention is a volume equalizer circuit whoseequalizing efiicency can be made independent of the ambient temperatureof the equalized station set. I i

Resistance elements suchas tungsten filaments which operate at a hightemperature usually become brittle; therefore, they are more easilysusceptible to failure from mechanical shock or with the appropriate.circuit connections being made to the transmitter and receiverterminals; The equalizer elements should preferably be made of materialhaving-a high negative temperature coefficient of resistance; heaterelement in the form of a coil of resistance wire or the like isthermally coupled to both of saidire-" sistance elements so as tovarythe resistance thereof by winding the coil around the resistanceelements. This heater element is connected to the substation circuitinsuch amanner that the 'heatingthereof varies directly with the stationoperating along an appropriate portion of thetemperature-resistancecharacteristic of the resistance elements, thechanges in the resistance of those element will besuch ast'o compensatefor the difference-intelephone' station set energizin current causedbydiiferent loop lengths 'toithewtelephoneswitching office where thestation.set;energizing current battery is usually located;Detailediexplanation of the operation andcharaoteiisticsof thermistors"may be de rived: from-the following articles: Varist'ors: theincharacteristic and uses, J'. A. Becker, Bell Laboratories, Record; vol;18; July 1940, pages 322-327;. Thermistors their characteristics andus.es,..*. 612th. Pearson, Bell Laboratories Record,

v.01. 19;.Decemher 1940, pages 106-111.

. In certain installations a: station set may be .subiect; torambient.temperature changes which would: affect-the; resistance. of the shuntcircuit -.c01nponents. .Suchtemperature changesare detrimental to;the:equalizationv efliciency of the" station set because theambienttemperatureof the thermal. resistance elements should: preferablybe-determinediby the heatenergy transfer from theahfiating element.andnot from. such factors as room temperature. The circuit of this-inventioneliminates. theseundesirable characteristics by providing for acompensating resistance ele-- ment to overcome the effects of stationset ambient temperature upon equalizationiefiiciency. In particularanegative temperature coefficient resistance element is oonnected inshunt with the heating element which heats the: receiver andtransmitterresistance elements. Changes invthe resistance of thiscompensation shunting: element dueto ambient station set temperatureproduce correspondin changes in the heating current of the heatingelement in such a magni- .:tudeas ,to, compensate for theveffect's ofthe'stationset. bierrtetemperature upon the thermal resistance:elements-which equalize the receiver and transmitter. 3

In-thedrarwing; the: equalizer circuit of thishYentiorr-dswshonnrsubstituted for the tungsten fil tmentieqnazlizercircuit in thestation' set circuit idisc-losediirriEigitof theapplication of Botsford cluded within dotted line enclosure I.

et a1. hereinbefore cited. Detailed reference will be made only to thecircuit components which comprise the equalizer circuit disclosed hereinor which closely cooperate with said equalizer circuit. The function andoperation of the circuit components shown schematically but not referredto are described in detail in the aforementioned application of Botsfordet ,al'.

The improved equalizer circuit, together with the station settransmitter and'receiver, is in- Transmitter 6 is energized by thedirect current from the line conductors through a circuit path whichincludes heating element 4. Heating element 4 is" thermally coupled tonegative temperature coefficient resistance elements 2 and 3, whichshunt the,- receiver and transmitter, respectively, through currentlimiting resistors I and 8, by close physical positioning or any otherarrangement for heat transfer known in the indirectly heated thermistorart. Negative temperature coefficienti element 9? shunts heating element4 and is to compensate for station set ambient temperature changes; Ifthe equalizer circuit components are embodied within a station setcradle or the like, it is desirable that compensation element 9 bethermally isolated from heating element 4 otherwise the temperaturecompensation efliciency will :be reduced.

The operation of the equalizer circuit is as follows: When the telephonehandset, which niechanically couples the transmitter 6 to thereswitchhoo-k contacts [0 and .3 indirect proportion to the current fiowthere'- through; On a short telephone loop=the heat energy transfer isgreater than that created by a longer loop; therefore, resistanceelements2-and -3- will assumea smaller resistance valu'e as compared tothatwhich would be assumed on a l-onger loop; Due to the increasedshunting effect of resistance element 3, the transmitter energizingcurrent passing through the transmitter is maintained at avaluesubstantially equal to that provided-by alonger loop connection. Theequalizationof the energizing currents equalize the average voicecurrent amplitudes generated by the transmitter. The reception signalamplitudes are'ke-pt substantially constant by the increased shunt-ingeffectof resistance element 2 on a short a loop as comparedtoa longloop.

Theinclusionofresistor 8-,- inseries with resistance element 3,facilitates design of resistance element 3 a-nd also ten'ds to preventarun-away conditien'whio'h-m-ight occur because lowering of .theresistance' ofelement 3 increases the current through -heating-element iwhich further lowers theresistance of resistance element 3. Resistor Iprevents;the-eXcessive-shunting of receiver 5 by resistance element; 2".

It-can be readily understoodthat-under certain conditionsr'of extremestation set ambient temperatures theequa'lization: of thestation setwill be: affectedexternal conditions or factors afiect the temperatureof thermal resista 'nce element's if and 3 other thanthetemperature-charigesdue toheat energy transfer from the ohmicresistance of saidtransmission line.

heating element 4. If the extreme ambient temperature persists during atelephone conversation, the average station set signal amplitude will beit higher or lower than a preascertained desired. level.v

These undesirable efiects on station set signal amplitudes can besubstantially reduced by l shunting heating element 4 with a negativetemperature resistance element 9. Resistance element 9 should not bethermally coupled to heatother arrangements may be devised by thoseskilled in the art without departing from the scope of the invention.

,What is claimed is: 1.' In a telephone circuit including a receiver,

, a transmitter and a direct-current energizing ment beingconnected inseries with said directcurrent energizing source, said transmitter, anda metallic telephone conductor so that the direct current, flowingthrough said heating element varies inversely to the physical length ofsaid tel- .ephoneconductor. l a

second resistance element-second resistor combinationshunting saidtransmitter, a heating element thermally coupled to said first andsecond resistance elements so as'to vary the temperature thereof inresponse to heat energy transfer thereto from said heating element, andmeans for varying the temperature of said heating element in response tothe amplitude of a direct-current potential applied to said substationcircuit by said energizing source.

4. In a telephone substation circuit including 'a receiver, atransmitter and a direct-current energizing source, a first resistanceelement hav- 1 ing a negative temperature coeflicient of resist- 2. In atelephone substation circuit including a receiver, a transmitter, adirect-current energizing source and a transmission line, a firstthermal resistance element whose resistance varies inversely to thetemperature thereof, said resistance element being in shunt connectionwith said receiver, a second thermal resistance element whose resistancevaries inversely to the temperature thereof, said second resistanceelement being in shunt connection with said transmitter, and a heatingelement thermally coupled to said first and second resistance elementsso as to vary the resistance thereof, said heating element beingconnected to said direct-current energizing source over saidtransmission line so that the heat transfer from said heating elementvaries inversely to 3. In a telephone substation circuit including areceiver, a transmitter and a direct-current energizing source, a firstresistance element having a negative temperature coefiicientofresistance and a first resistor being connected in series with oneanother, said first resistance elementfirst resise tor combinationshunting said receiver, a second anceand a first resistor beingconnected in series with one another, said first resistanceelement-first resistor combination shunting said receiver, a secondresistance element having a 1 negative temperature coeflicient ofresistance amplitude of a direct-current potential applied 1 to saidsubstation circuit by said energizing source, and a third resistanceelement having a negative temperature coefficient of resistance shuntingsaid heating element so as to compensate for the effect of ambienttemperature upon said first and second resistance elements.

5. In combination, a telephone connecting loop and a volume equalizertelephone circuit comprising a receiver, a transmitter, a negativetemperature coefficient resistance element comprising a direct-currentshunting path around said receiver, a negative temperaturecoefiicientresistance element comprising a direct-current shunting path around saidtransmitter, and means controlled by the ohmic resistance of said loopfor controlling the temperature of both of said elements. 7

6. In combination, a telephone connecting loop and a volume equalizerfor compensating for the different interconnecting loop lengths betweendiiferent subscriber stations and the telephone switching office servingsaid stations, comprising a transmitter, a receiver, a negativetemperature coefiicient resistance element shunting said receiver, anegative temperature coeificient resistance element shunting saidtransmitter, and Y a heating element controlled by the ohmic resistanceof said loop for controlling the temperature of both of said resistanceelements.

- ESTILL I. GREEN.

References Cited in the file of this patent UNITED STATES PATENTS

